In this paper, we analyze the security of a new stream cipher-COSvd(2,128). This cipher was proposed by E. Filiol et al. at the ECRYPT SASC’2004 (The State of the Art of Stream Ciphers). It uses clock-controlled non-linear feedback registers together with an S-box controlled by a chaotic sequence and was claimed to prevent any existing at- tacks. However, our analysis shows that there are some serious security flaws in the de- sign of the S-box, resulting in heavy biased byte distribution in the keystream. In some broadcast applications, this flaw will cause a ciphertext-only attack with high success rate. Besides, there are also many security flaws in other parts of the cipher. We point out these flaws one by one and develop a divide-and-conquer attack to recover the secret keys from O(226)-byte known plaintext with success rate 93.4597% and complexity O(2113), which is much lower than 2512, the complexity of exhaustive search.
Ω-protocols, introduced by Garay, Mackenzie and Yang, is a variant of S-protocols with online extractor which is a useful tool to overcome the nest effect in concur- rent scenario. In this work, we construct an Ω-protocol for Hamiltonian cycle prob- lem, and therefore, it allows us to present Ω-protocol for any NP relation. For most general NP relations, our construction of Ω-protocols is much more efficient than the informal one described by Garay et al. and we believe that the method for our construction may be of independent interest.
